The present invention relates to a method for use in making semiconductor devices and, more specifically, is directed to a method particularly adapted for the manufacture of photovoltaic solar energy cells.
In a prior patent, U.S. Pat. No. 3,895,975, on which I am named as patentee, a method for making solar cells and other semiconductor devices is disclosed whereby problems associated with back surface diffusion of impurities and stress formation in semiconductor device manufacture are significantly reduced while at the same time two junctions having improved electrical characteristics were formed within the device. That patent is entitled, Method for the Post-Alloy Diffusion of Impurities into a Semiconductor.
The method disclosed in that patent is unclear and is described in differing and conflicting ways in the specification. However, it is apparent that it includes placing a layer of alloying material, such as aluminum, on the back surface of a silicon wafer doped with an impurity of one conductivity type, then heating the wafer to a temperature at which an alloy of the alloying and semiconductor materials will be formed. At least as recited in the claims of that patent, the alloying material also diffuses into the semiconductor material. Thereafter, an impurity of the opposite conductivity type, such as phosphorus, is diffused into the front surface of the wafer.
While the method set forth above can produce semiconductor devices of satisfactory quality, the method is unsatisfactory in at least one important respect for use in the high-volume production of semiconductor devices, particularly photovoltaic cells. Thus, the requirement of the patented method that the layer of alloy material first be alloyed with the semiconductor material and subsequently diffused with an impurity from a gaseous medium necessarily requires that either the method be limited to a batch-type operation, or that expensive and complicated equipment for a continuous-type operation must be provided. In explanation, where two different atmospheres are to be used in a continuous production line employing belts to convey the wafers, two furnaces would be necessary, one having an inert atmosphere, the second atmosphere containing the diffusant gas. Multiple furnaces and, quite possibly, the need for multiple conveyor belts present a complication and expense that is definitely undesirable.